Photographic film comprises a light sensitive coating on a polymeric support. Since about 1930 the support material of choice has been cellulose triacetate (sometimes called cellulose acetate). Once the film had been used it is possible to recover the support material for reuse of the polymer by separating the light sensitive coating from the polymer support and then treating the support. Numerous ways have been devised for recovery of cellulose acetate from film supports.
Research Disclosure Item 12629, published October 1974, Research Disclosure Item 19634, published August, 1980, U.S. Pat. No. 1,844,711 dated Feb. 9, 1932 and U.S. Pat. No. 3,956,088 dated May 11, 1976 describe some techniques for treating film supports to recover polymer from them.
In a series of Russian patent abstracts (SU 1191876, 1205118, 1216766, 1282063, 1282064, and 1418639) published between 1985 and 1988, Tereshkova describes recovery of cellulose triacetate film using combinations of various oxidizing and reducing agents.
In Japanese Kokai 04-350856, dated Dec. 4, 1992 and 05-281686, dated Oct. 29, 1993 there is described recovery of cellulose esters using a halogen oxide oxidizing agent in combination with either a manganese or peroxide oxidizing agent. These processes use organic solvents and/or hydrogen peroxide, both of which are undesirable in an operation run on a commercial scale.
A common commercial procedure for recovering cellulose triacetate polymer from used motion picture film involves size reduction followed by treatment with sodium hydroxide, potassium permanganate and sulfuric acid, sodium metabisulfite and washing with water. The size reduction gets the film in a form that it can be readily handled and acted upon by the chemical agents which follow. Treatment with sodium hydroxide hydrolizes the gelatin in the coatings on the film support and prepares the various layers for oxidation and removal. Treatment with potassium permanganate and sulfuric acid oxidizes the components of the various layers. Treatment with sodium metabisulfite neutralizes the oxidizing agents and bleaches any brown stain attributed to manganese dioxide. Water washing removes the reaction by products and suspended particles from the cellulose triacetate slurry. The freshly washed support is recovered by drying.
While this process provides cellulose triacetate which is useful for a number of applications, a problem with it is that with some sources of film base, such as that which has been used as motion picture film, the process does not yield a product that is sufficiently colorless that it can be used to prepare photographic film base. Rather, the cellulose acetate resulting from this process has a yellow to brown color which renders the polymer unsuitable for use as a support for photographic film. None of the art relating to the recovery of cellulose acetate provides a suggestion as to how to eliminate the color in the recovered product.
We have determined that the color is attributable to dye and iron compounds that are present in the photographic film and that these compounds are not removed or destroyed by the recovery processes heretofore used. We have found that cellulose acetate of acceptable color purity can be prepared by a process which has as one significant feature the use of a separate hypochlorite oxidizing step. We also have found that elimination of the sodium hydroxide treatment improves the color of the recovered product.